The present invention relates to a scanning electron microscope for observing a structure and the like of a specimen to be observed by irradiating an electron beam on the object to be observed and detecting secondary charged particles generated therefrom, such as secondary electrons and reflected electrons, and a system for inspecting semiconductor devices and more particularly, to a technology effectively applicable to an inspection technique for observing minute circuit patterns formed on a semiconductor wafer and the like and defects and the like generated in the circuit patterns.
In order to fabricate semiconductor products such as microprocessors and memories at a high yield, it is important to monitor fabrication status in steps of semiconductor fabrication by monitoring whether a circuit pattern to be formed on a semiconductor wafer is produced with predetermined shape and size and observing whether a defect (such as deposition of foreign matters or pattern thinning) is generated on the pattern and to study causes to thereby take countermeasures in the presence of problems.
For the sake of the process monitoring, pattern observation based on an optical microscope has hitherto been practiced but presently, the width of pattern wiring formed on a semiconductor wafer is of an order of hundred nanometers or less and recently, a scanning electron microscope capable of observing a fine structure of the nanometer order has been used for the purpose of inspecting circuit patterns.
A digital image acquisition process in the scanning electron microscope as above includes a step of irradiating a focused primary electron beam on a specimen while scanning the beam two-dimensionally, a step of detecting secondary charged particles such as secondary electrons and reflected electrons generated from an irradiated site and a step of sampling a detected signal to convert it into digital values. By arranging digital signals obtained through these steps on a two-dimensional arrangement such that these digital signals are arranged at the same positions as those scanned with the primary electron beam on the specimen, a two-dimensional digital image can be prepared.
The image quality of the digital image obtained with the scanning electron microscope has the influence upon the visibility when the image is displayed and upon the performance of an inspection process conducted using the image and therefore, digital image processing for improving the image quality is usually applied to the acquired image in advance of display and process.
For example, in order to improve the quality of low S/N and low contrast picture, procedure is taken including a process for improving the S/N and contrast by means of a smoothing filter and a frame addition process in which digital images are acquired at the same site of a specimen and average values of individual plural pixels are used as pixel values of an ultimate output image.
As another method, JP-A-2003-331769, for example, discloses a technique according to which an acquired scanning electron microscope image is subjected to a de-convolution process so as to remove a blur component of the image caused by the fact that an incident electron beam has a finite beam diameter, that is, the incident beam has energy distribution of a certain width.
Incidentally, inventors of the present invention have studied technologies of scanning electron microscope as above to clarify the following.
For example, in the image processing method concerning the background of the invention, digital images of the predetermined number of pixels are acquired and thereafter various procedures for improving the image quality are executed. Here, by taking acquisition of a one-dimensional signal generated from a specimen, for instance, the relation between the background art and the frequency information the acquired digital data has will be described.
FIG. 14A depicts the behavior of sampling an electron beam signal 201 generated from a specimen at constant sampling intervals 206, that is, at sampling timing positions 202. One of the sampling intervals 206 signifies the size of one pixel in a digital image. In the scanning electron microscope, the one interval is definitely determined from the number of pixels and the magnification of the digital image to be acquired.
FIG. 14B diagrammatically expresses the electron beam signal 201 generated from the specimen and its frequency in frequency space. In FIG. 14B, the sampling frequency or rate in FIG. 14A is expressed by an aliasing frequency 204 and an aliasing signal 205 due to the aliasing takes place in a signal component 203 in frequency space of the obtained electron beam signal 201 generated from the specimen. In typical case, this corresponds to the fact that the electron beam signal 201 generated from the specimen has a sophisticated structure and contains a frequency component higher than the sampling intervals 206. In the image quality improvement process concerned with the background art, the smoothing filtering and other image quality improving processes are applied to the image acquired at the sampling intervals 206 and this signifies that signal processing is conducted within the range of a frequency region 207 in FIG. 14B. It is therefore meant by the image restoring process according to the background art that the signal processing is conducted by noticing only the frequency region 207 without considering a region 208 and as a result, the sufficient image quality improving effect cannot sometimes be expected.